Distribution of La Crosse (LAC) and similar arboviruses will be studied in hardwood deciduous forests in suburban and rural areas where children are being exposed to California encephalitis. Comparative rates of past infection in forest-dwelling mammals will be measured by antibodies detected by micro-neutralization tests in BHK-21 cells. Prospective studies with sera from marked and recaptured chipmunks, squirrels, and sentinel rabbits will provide data on levels of continuing virus activity. Persons infected in these areas will be included in epidemiologic studies to learn more about the circumstances of La Crosse virus infection and disease. Aedes triseriatus and other vector populations, determined by aspiration collections and oviposition traps, will be correlated with weather and other environmental factors. Distribution of LAC virus in insects and other tissues will be detected by fluorescent antibody and mouse inoculation techniques. Studies of overwintering mechanisms for the virus in Aedes triseriatus will be continued, further defining the frequency and distribution of natural basal tree-hole breeding sites, and any other sources of LAC virus necessary for maintenance in these forested areas where it occurs year after year.